Enter a discount or customer share code in the checkout code field when available. Apply it and review the final total before paying. If a link filled it in for you, still check that checkout accepted it.
If it is refused
Check the code for typing errors and review its conditions, such as eligible plans, minimum spend, expiry or previous use. Use the actual checkout message rather than assuming all codes have the same rules.
A partner tracking link and a discount code are not necessarily the same thing. A referral can record where a customer came from without reducing the price.
After payment
Do not assume a code can be added retrospectively. Contact support with the order and code if something looks wrong.
Rewards, where offered, follow the terms of that specific programme. A checkout discount does not by itself confirm that another person has earned or received a reward.